This invention relates generally to planar contact array switches and more particularly to a planar contact array switch which includes charge conducting paths for conducting electrostatic discharge away from the membrane switch contacts to a ground plane.
A common type of data input device for an electronic circuit is a planar contact array switch. Although the specific construction details of planar contact array switches vary from manufacturer to manufacturer, generally the switch is comprised of a printed circuit board which carries at least one conductive switch contact path, the other switch contact path being adjacent thereto. Overlying the conductive switch contact path on the circuit board is a conductive member, which may be domed in shape. The conductive member is in contact with the other of the switch contact paths and sandwiched between the printed circuit board and an insulative overlay. When the conductive member is urged against the switch contact path on the printed circuit board, such as occurs when a fingertip force is applied to the area of the insulative overlay above the conductive member, then a completed electrical circuit is created between the switch contact paths.
Planar contact array switches are frequently used in industrial environments such as the shop floor of a manufacturing facility since the insulative overlay serves to protect the delicate switch contact paths from industrial contaminants, such as oil, grease or the like, which might otherwise interfere with switch operation. Usually, the insulative overlay is formed from a polycarbonate or polyester plastic so that the insulative overlay may be wiped clean after the inadvertent application of a contaminant as will often occur when the switch is activated by a human operator on the shop floor.
While the insulative overlay associated with this type of planar contact array switch effectively shields the underlying conductive switch contact path from exposure to industrial contaminants, the insulative switch overlay can be penetrated by the high voltage electrostatic discharges which may be transferred from a human operator upon the application of a contact force to the overlay to activate the switch. The electronic circuit components connected to the conductive switch contact paths of the switch are often very sensitive to electrostatic discharges and may be damaged or destroyed if an electrostatic discharge applied to the overlay penetrates the overlay and is conducted to the circuit components through one of the conductive switch contact paths.
In an effort to reduce the likelihood of electronic component damage due to electrostatic discharge, planar contact array switches are often provided with a conductive ground path adjacent to the switch contact area so as to bound the area occupied by the switch contact paths. The ground path serves to conduct any electrostatic discharges away from the conductive switch contact paths and carry them to circuit ground. However, an electrostatic discharge which penetrates the insulative overlay within the area bounded by the ground path may, in fact, "jump" to an adjacent one of the interior conductive switch contact paths before reaching the conductive ground path. Thus, the perimeter ground path may not always be effective to carry electrostatic discharges away from the interior switch contact paths on the printed circuit board.
In an effort to overcome this disadvantage, the present invention is directed to a planar contact array switch in which electrostatic discharges penetrating the insultative overlay within the boundary of the conductive ground path are carried to the conductive ground path rather than allowed to jump to the conductive switch contact paths.